OK I know the NMRA has their reccomended weights for various rolling stock, I have heard more than a few times that model ocos are able to pull more cars than their protortype. I'm looking to do a thought experiment on what would give more accurate "trailing tonnage" for our models.
Mind you I'm not looking to turn the reccomended practice on its head, it works well and for most modelers its just fine. But I feel that the RP may be out of date considering the quality of models today vs from the 50's and 60's.
Lets be honest, the typical diesel loco in those days had a single powered truck. Many steam models had the drive in the tender. Most were very lightly weighted. All add up to poor pulling power.
Todays diesels are pretty much all wheel drive and tender drive steam is pretty much non existant. Newer locos simply ahve more pulling power than they used to.
Im curious as to how prototype loco weights compare to prottype trailing tonnage and how that can be "scaled down" to modeling. I know theres been many discussions on how weight doesn't scale very well, and I belive there will never be an acceptable answer to that question. So i'm thinking of more proportional weights.
I think and example of what I'm thinking would give the clearest explanation, so:
lets say a 4 axle diesel weighs 150 tons, and can pull a trailing tonnage of 1500 tons. Thats 10 to 1.
Lets say a HO model of that loco weighs 15 ounces, how close to 150 ounces of trailing tonnage does it pull? Are we still close to 10 to1?
The flip side to this becomes car weights, so a loaded 50 ton hopper weighs 50 tons. If the loco math stays the same (10 tons equals 1 ounce) then a loaded model should weigh 5 ounces.
And of course the flip side to loaded cars is empties. What does an empty 50 ton hopper weigh? What should its model weigh?
While the weights of locos (prototype and model) are pretty easily found on the internet, finding good trailing tonnage numbers is a bit more difficult. Obviously those numbers have a huge number of variables in the real world due to grades, curves etc etc etc, but someone somewhere should have some knowledge of what a given loco could pull on a given railroad. I'm thinking A former UP or SF employee caould share some info for a long flat fairly straight division of one of them.
Of course a thought experiment is a fun bag of what ifs and why nots, putting it into practice would be quite enlightening and entertaining. This could add a whole nother world of challenges for the operations type modelers (train handling changes for the engineers, blocking cars for the yard crews, the dispatcher now has to take train weight into consideration when deciding which to put in the hole etc etc)
Modeling the Cleveland and Pittsburgh during the PennCentral era starting on the Cleveland lakefront and ending in Mingo junction
The purpose of NMRA RP 20.1 Car Weights, is keeping trains on the track, not scaling model weight to match prototype weight. It is well known that a light car inbetween heavier cars can get pulled off the track on curves (stringlining). The solution is to weight all the cars about the same. At the time RP-20.1 was published models contained a good deal of die cast metal and were in general, heavier than modern molded plastic models. It being easier to weight up light cars than to lighten heavy cars, the standard was set a bit heavier than it might otherwise have been. And since model rolling stock lasts a long time (I run some cars that are 50 years old) nobody wants to change the standard.
David Starr www.newsnorthwoods.blogspot.com
Dstarr, like I said, I'm not looking to overrun the RP, just looking atthe situation from a different angle. And if the RP works for you go for it. And yes I know the RP is more to help tracking, hence I pointed out that follwing this thought would make train makeup a more serious consideration for those who choose to go this route instead.
Your point about older cars containg metal bodies and frames (die cast or brass or lead) and newer ones being plastic emphasises my point that RP may be out of date.
Yes the RP works, I'm just considering an alternative.
I know people that add weight to NMRA and others who have done heavey cars or run as they come. All work fine if the track work is solid (of course one would limit some combos on the as they come bunch. You are right that things have changed but not for everyone, most have tighter turns and steaper grades than real life (mine are 18" radius curves and 4% grade up to one yard with an even steeper grade going back down to the mainline and my layout is 15'x30' but this section was oringinally loging the rest of the layout being flat with elevation features carved in or built up).
mel, interesting link. Gives me an idea about looking up tractive effort for a loco and working back to trailing tonnage.
Hmm OK this got moved.
RRebell, of course models run much tighter curves that the prototype, we just don't have the space for what would be realistic. But we do tend to follow the prototype in regards to grades.
Yes our tight curves increase the drag on the train and the risk of stringlines. Thats what the RP is for. And I realize that making a wider spread in car weights will amplify the situation. This adds to the challenge that operations types wish to emulate.
Im not saying its a good idea or bad idea, that would be up to the individual modeler. I'm just exploring a possible way to emulate the real world.
I agree that a layout should be run with some thought to what sort of power a "real" railroad would assign to the train lengths we feature, but the problem is that as a rule our trains tend to be so short that most railroads would hold it in the yard until more cars can be added. As a rule model trains over overpowered, steam or diesel.
Also, having "been there" I cannot totally agree with the OP's basic premises about model railroad history. There were plenty of diesel models in the 50s and 60s that had all axle drive, thinking back to the Varney F3 that was a "classic" on many layouts, weighed a ton, and pulled like the devil. Ditto for Hobbytown of Boston, whether with their own heavy metal shells, Cary metal shells, or Athearn plastic shells. Even the Athearn rubber band drive F7 was all axle powered, and their gear driven line was all-axle. Penn Line had a low end F unit with one truck powered - but it had a huge lead weight over that truck and pulled rather well. Yes many of the AHM trainset quality diesels had one powered truck (with traction tires but not much weight) but then they were intended to pull AHM trainset quality (and thus uniformly weighted) freight cars on small oval layouts featuring fairly short trains. As for steam locomotives, apart from Mantua's General, and some of the Rivarossi/AHM smaller engines (and for a brief while, their IHB 0-8-0), tender drive was fairly rare as I recall. I guess Fleischmann had tender drive for some stuff too.
And as far as rolling quality of trucks goes, I am not aware of any current trucks on the market that roll measurably better than the Lindbergh trucks circa 1962. Central Valley trucks rolled well too. Red Ball trucks ... eh, not so well.
And then let's not forget that a prototype locomotive would likely be unable to pull ANY cars at realistic speeds through the equivalent of our model railroad curves because the darn thing would be likely on the ground.
If a prototype 2-8-0 was good for 30 to 50 cars, and a prototype Mikado might have been assigned to a 75 car train, seems to me there are not many models sold today of those steam engines that can handle loads of that size, using freight cars made today or made years back for that matter, regardless of weight. But even if they could - how many guys (versus clubs) run trains of that length?
If you go back to the 1950s there was not much said about NMRA standards for car weight - rarely if ever mentioned in MR product reviews for example. But a very real problem existed that needed to be dealt with. You had plenty of freight cars of the Model Die Casting, Ulrich, Red Ball all-cast-pot metal variety which were very heavy. Also the Athearn and Varney steel bodied freight cars with wood "innards" which were fairly heavy. Standard kits from Silver Streak, Red Ball, Ambroid, and such which were largely of wood with some white metal details, and not particularly heavy. And a whole host of the then-new all plastic cars from Varney, Hobbyline, Kurtz Kraft and Athearn, which often came without weights at all and were usually feather weight. The Varney ore cars in particular weighed practically nothing beyond what the trucks and couplers weighed. And then there were cars that guys scratchbuild of available materials, but mostly wood with pot metal details, which could be all over the map in weight.
(I am aware that the prototype railroads also had concerns about light cars being placed at the head end of trains, so that a there was a good prototype reason to put an empty Varney or Hobbyline plastic flat car that weighed nearly nothing back towards the caboose. But a Model Die Casting boxcar and a Hobbyline boxcar weighed wildly different amounts. There was no prototype based reason to treat them differently in train placement.)
Trying to run those cars together in one train, regardless of trucks or locomotive, was an adventure in derailments, and that was true whether the layout had trainset curves or more generous curves. Not to mention the (brief) popularity of the then trendy X2F horn-hook coupler (enthusiastically called the NMRA coupler by nearly everyone including the NMRA leadership until it became clear they were flawed and were never going to be approved by NMRA membership at which time the NMRA tried very hard to extricate its name from the design), which tended to push cars to the side as it was.
The situation cried out for standards. I do not think anyone, even the NMRA technical committees, ever claimed that NMRA standard weight is in some way the "only" perfect way to weigh a car. What was needed was a practical weight standard based on car length(s) so that wood, metal, plastic, and hybrid mixes of the three could continue to be used in car construction and run on layouts in a prototypical manner. The materials we use these days, almost all plastic, would permit lighter cars but now that there is a trend to put decoders and sound systems in certain freight cars such as mechanical reefers, yet another factor has been thrown into the mix in terms of what a good practical uniform weight standard should or can be.
Some clubs do indeed have their own standards for car weight and vigorously enforce those standards before any car is permitted on their track. And they report good results. If you have that sort of discipline then NMRA or other standards can be disregarded. But with more RTR cars that are the very devil to try to open up to either remove or add weight, my hunch is that standard weights such as NMRA's are as important as ever, regardless of what pulls our trains, because we have seen what happens when the materials used, and ONLY the materials used, dictate car weight, and it is not a very happy place.
Dave Nelson
ruderunner mel, interesting link. Gives me an idea about looking up tractive effort for a loco and working back to trailing tonnage.
ruderunnerOK I know the NMRA has their reccomended weights for various rolling stock, I have heard more than a few times that model ocos are able to pull more cars than their protortype.
Actually, this isn't the case. The laws of scaling mean that model locos aren't are heavy relative to size as the 1:1.
Yes, there are other factors at work, like MUing diesels, sharp curves, etc. But the fundamental fact is are locos are nowhere near as heavy as they would be if weight/mass scaled the same way that dimensions do.
No, you're not stuck with the NMRA's recommendation, but it's a very good thing to understand it and keep in mind. Narrowgaugers often fudge things by going lighter than NMRA recommendations, but you MUST be consistent still to make this work, as several have already observed.
Mike Lehman
Urbana, IL
on the one hand, it's not how heavy the car is, but how much force is needed to make it move and on level ground this means overcoming friction
Years ago I though I read it is 8 lbs per ton for a typcial freight car. A 50 ton hopper would require 400 lbs of force to overcome friction.
The excess force, that greater than friction, then accellerates the car up to speed.
I happen to know that a Reading I-10 2-8-0 has a tractive force of 55,000 lbs. Dividing this by 8 lbs is 6800 tons or roughy 100 70 ton cars. But this leave nothing for grades.
on the other hand is that when there is a grade, a fraction of the weight of the car must also be be overcome. If I did the trig correcrly, a 1% grade requires an additional 35 lb per ton. Roughly double that for 2%.
This topic came up last year Prototypical horsepower versus actual pulling power of models
greg - Philadelphia & Reading / Reading
dstarrThe purpose of NMRA RP 20.1 Car Weights, is keeping trains on the track, not scaling model weight to match prototype weight. It is well known that a light car inbetween heavier cars can get pulled off the track on curves (stringlining). The solution is to weight all the cars about the same. At the time RP-20.1 was published models contained a good deal of die cast metal and were in general, heavier than modern molded plastic models. It being easier to weight up light cars than to lighten heavy cars, the standard was set a bit heavier than it might otherwise have been. And since model rolling stock lasts a long time (I run some cars that are 50 years old) nobody wants to change the standard.
Let's look at the real reason..Cars at one tine was made from wood with paper sides or printed on road names these cars did not have any weight other then the trucks and the trucks rolled like the brakes was set..The NMRA worked their math and decided RP20.1 would work.Varney cars was light as well since there was no added weight. Walthers freight and passenger car kits came without weight.Same for the Hobbyline plastic car kits no weights included. We needed RP20.1 back then but,it never was widely received by the modelers of that time..
Today its outdated due to the various modern car lengths-some long cars will be much heavier then a 50 boxcar.Then we have stacks and the front runners and 89' piggy back cars.
Be that as it may we still have RP20.1 as a guide line-common sense should come into play when weighing our modern cars.
As far as they famous cry of "stringlining" the only time I've seen that was when a super long 50 plus car train was ran around sharp 24" curves on a 2% grade. The average 20-30 car trains had no issues traversing this area on the club's layout.. I been told that section of the layout has been redone since I was a member several years ago.
I have read on various forums modelers whining about their new DCC/Sound engine can pull only 6 cars up their 2% grade.
The first question I think of how much did you weigh your cars? Some day I may even ask that question.
I'm experimenting with 3 pieces of stick on weight over each truck equipped with IM wheelsets. Not because of derailments but,for increase rollibility.
Larry
Conductor.
Summerset Ry.
"Stay Alert, Don't get hurt Safety First!"
ruderunnerLets be honest, the typical diesel loco in those days had a single powered truck. Many steam models had the drive in the tender. Most were very lightly weighted. All add up to poor pulling power.
All depended on brand.Brass diesels had 8 wheel drive and steam used all of its drivers for pulling.
The popular Hobbtown drive used a 8 wheel drive and Athearn did as well on their earilest diesels-these was available along side of their Hi Fi drives.
There was another brand that used a 8 wheel drive-Revelle?
Brass wheels on brass track seem to pull more then nickel silver wheels on nickel silver track.
Disclaimer - I have not read all of the replies above.
A few simple points:
Our models have dramaticly different factors of adheasion and coefficients of friction - a direct comparison cannot be made.
I started in this hobby in 1967, and all the diesel locos I had were all wheel drive, Varney, Athearn, etc - but admittedly I did not have TYCO train set stuff.........
Having worked in the model train business as far back as 1970, very few steam locos ever had tender drive..........
Most of my rolling stock is slightly lighter then NMRA RP, but most also have metal sprung/equalized trucks with metal wheels/axles, keeping weight down low for better tracking.
Generally, model diesels will equal or out pull their prototypes, but the reverse is generally true with model steam, even after adding weight, etc - brass, die cast or otherwise. Again issues of adheasion and friction not the same.
I pull long trains, 40, 50 cars and more no troubles. Two steamers or three and four unit diesels, large curves, etc.........
My cars are generally very free rolling - most have Kadee sprung metal trucks refitted with Intermountain wheelsets, a combination which in my testing out rolls any rigid frame plastic truck and tracks better.
I saw a 30% increase in the pulling power of my locos with the implimentation of that as my standard truck for most freight equipment.
And again, most of my cars are weighted to about 80% of the NMRA RP.
Sheldon
ruderunnerIs there a correlation between the MRR drawbar pull and tractive effort of the prototype?
i doubt manufacturers are scaling the motor in their models to match tractive force of protoypes which also depends on weight on drivers.
the same DC motor is often used in an 0-4-0 as well as a 2-8-8-2
i've never heard of a model that stalls.
my point is that a weighted model can far exceed the tractive force of the prototype. Model locomotives are often too light.
If we're doing a thought experiment . . .
Anyone here ever watch old-timey war movies? In the old days (before CGI), moviemakers would film scale model battleships and destroyers and whatnot in a swimming pool or something. Ever notice how the water just never looked right? There's a reason for that. You can scale down size, you can scale down weight, but you can't scale down force.
LINK to SNSR Blog
Model steam locos vary greatly in their pulling ablity, and there are lots of complex reasons depending on the particular model.
But model steam is also subject to many of the drawbacks of prototype steam.
Steam locos loose traction on curves faster/sooner than diesels, the sharper the curve, the longer the rigid wheel base, the faster this happens.
Steam locos loose power or grades sooner than diesels as well.
Our models, steam or diesel do better on grades than the prototype. A grade over 2% is generally a 50% loss of tonnage ablity with average rolling model equipment.
On the prototype, especially with steam, a 2% grade reduces tonnage by as much as 80%. That's why it took four 2-8-8-4's to move the same 70 cars over the Appalachian summit (ruling grade about 2.25%) that only required two 2-8-2's to move from Baltimore to Burnswick, MD - a ruling grade of 1/2%. And one of those Mikes could easily handle those 70 cars if the grade had been zero.........
Again, when I went to free rolling metal trucks, the pulling power of my fleet went up 30%, steam and diesel.
I'm not necessarily trying to scale it down. I'd like to keep it in proportion. Let me ask this, say a scale loco Gan pull twice as many NMRA weight cars as the prototype can. Would that not indicate that the NMRA weight should be doubled?
Again, I'm trying to leave grades and curves out for the time being. I'd like to start with a basic proof of concept and then expand into things like that.
And if it ever does get to the point, perhaps some club or someone with a large enough home layout could try to put the findings or theory to the test.
ruderunnerLet me ask this, say a scale loco Gan pull twice as many NMRA weight cars as the prototype can. Would that not indicate that the NMRA weight should be doubled?
why not reduce/limit the pulling power of the locomotive?
ruderunner I'm not necessarily trying to scale it down. I'd like to keep it in proportion. Let me ask this, say a scale loco Gan pull twice as many NMRA weight cars as the prototype can. Would that not indicate that the NMRA weight should be doubled? Again, I'm trying to leave grades and curves out for the time being. I'd like to start with a basic proof of concept and then expand into things like that. And if it ever does get to the point, perhaps some club or someone with a large enough home layout could try to put the findings or theory to the test.
OK, on the prototype every line has known ruling grades and curves, the dispatchers and yard master know what tonnages require what power, they build trains and assign locos accordingly.
Why not just figure out the pulling power of your varoius locos, assign loaded and empty tonnages to your rolling stock on paper, and build prototypical trains?
That is what I do.
As explained above, it took two Mikes to move a B&O train of 70 cars, likely about 3,000 tons, from Baltimore to Brunswick, on a 1/2% ruling grade.
I typically put two locos of that general size on the front of a 40-50 car train on my layout.
Close enough for me, looks pretty realistic, the locos can handle the 50 cars on my 36" radius curves and less than 2% grades.........
Same train, diesel powered, would have three to four units, first generation, 1500 HP each (F units, FA units, GP's, etc). It was generally considered that a Mike was about equal to 3000 HP......
I know this, one of my Mikes cannot pull 50 cars comfortably around the layout.....two can.
by scaling prototype weights by 87^3 (~650,000), get the following
50-ton car is 2.4 oz and 70 ton car is 3.4 oz. These aren't terribly far off from the NMRA guidelines of 1 oz + 0.5 oz per 1" (e.g. 4" is 3oz)
another question is how does the friction of model freight cars compare to prototype cars. Greater wheel friction would be equal to adding weight.
From Armstrong's, The Railroad, What it is, What it Does, a 6-axle diesel weighs 380,000 lbs == 9.2 oz and has a tractive force of 50,000 lbs at 20 mph == 1.2 oz, and half that at 30 mph
the following shows some measurements for HO scale drawbar force vs weight. But considering that 1 oz == 28 grams and that the charts indicates drawbar force in grams ranging from roughly 30 to 280, the minimium drawbar force in the chart is roughly 1oz.
in other words, the scaled prototype draw force of 1.2 oz is near the bottom of the chart. Most model locomotoives far exceed the prototype.
another thing to consider is the number of cars in a prototype train (~100) vs models